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Vintage Radio
By Associate Professor Graham Parslow
Restoring a pile of
hydrated ferric oxide
This was once HMV’s C13C 5-valve mantel radio
Why would you want to restore an “unrestorable” radio when you
already have a number of similar radios by the same manufacturer and
with the same valve line-up? It all comes down to the cabinet. There
were so many cabinet styles and colours and some are more interesting
than others. And of course, there was the challenge...
The most memorable aspect of this
radio was how I came to acquire it. It
was on a seller’s table at an Historical
Radio Society of Australia (HRSA)
meeting in Melbourne.
It was late in the day as I passed a
table manned by HRSA vice president
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Mike Osborne. With considerable good
humour, Mike solicited me to purchase
this radio.
He suggested that I should acquire
it as a challenge to my reputation as
patron saint of lost-cause radios. As an
aside, Saint Jude the apostle is held to
Celebrating 30 Years
be the patron saint of other lost causes.
This radio was so far lost and degraded that the old song “get out of
here with that boom-de-boom and take
it down below” came to mind. As Mike
persisted, the asking price came down
until in desperation I was offered $2
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You can see above that this shattered Bakelite cabinet looks almost beyond
repair, but it hides a chassis that has decayed to the point that it can never
be restored.
While shown at right is the underside view of the chassis, with various
components having been shed over time.
to take it away. I accepted. True to his
word, Mike handed over $2 but I declined and paid him $2 for his hard
work in selling the radio.
It looked like a pile of rubbish and it
was. The cabinet was badly fractured
and that was only the start. And while
I had paid the princely sum of $2 for
ten minutes of banter with Mike Osborne, I was really just saving him the
trouble of carting it home and putting
it in the bin. I was certainly not motivated to restore it.
I thought it looked liked a generic
no-brand type that various chain stores
marketed under brands of convenience, at the time. However, the fluted side moulding on the cabinet did
give a stylistic clue that it might be a
HMV model.
Just as a matter of curiosity, I sent an
email to several HRSA members who
might be able to recognise it and sure
enough, Jim Eason (HRSA treasurer)
came back with the correct identity.
Fortunately, a good example of the
C13C was shown online in Ernst Erb’s
Radiomuseum in Switzerland (www.
radiomuseum.org).
The radio stayed in the box that I had
brought it home in for quite some time
before I ventured to have another look
at it. Once it was out of the chassis, it
was clear that I had purchased a badly
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deteriorated Bakelite cabinet containing a kilogram or so of hydrated ferric
oxide and other debris!
And the reason the ferric oxide was
hydrated was that the radio had evidently been partly submerged in water for some years. The water had destroyed every component under the
chassis except for some coils and resistors; hardly a good starting point for
an electrical restoration.
So the chassis was definitely not a
prospect for full restoration and that is
an understatement. I have numerous
working radios of this general type so
I knew what it would sound like. I do
have other radios which are far more
deserving of full restoration. But perhaps this was a case for a display-only
restoration...
But there is a compelling temptation
among most radio collectors (myself
included) to take a peek at the back
of a radio, to see the valves and general layout.
Because of the information on the
Radiomuseum website, perhaps I
would be able to reproduce labels and
add components, to give a cursory simulation of a working radio.
After all, the human eye is easily
deceived. We frequently perceive an
object as simply another example of
something familiar. We fill in details
that are not there and we can easily
miss anomalies. Film makers and magicians are well aware of this.
In James Cameron’s 1997 film
Titanic one scene is taken on a deck
of the majestic ship but if you freeze
the frame and look to the far left, you
can see where the mock-up ends and
the studio begins.
Few people ever noticed that mis-
The screen-printed glass dial was virtually the only component that survived
years of immersion in water. Maybe the mud preserved it.
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January 2018 89
This is the chassis after it had been washed. Note the remains of the
loudspeaker and the exposed windings of the transformer.
take or any of the other numerous visual errors in that film (check them out
at https://youtu.be/8-JXpxr0fzg).
And this radio will certainly never
pass close scrutiny. I must admit to
having serious doubts about whether I
could even justify the work required to
make it worthwhile as a display-only
set when I took it out of the cabinet, as
the chassis lay on my bench dropping
rust and other miscellaneous detritus.
There was virtually nothing that was
recognisable on the underside.
The top of the chassis was similarly discouraging. The tuning capacitor
had evidently completely dissolved
and just a rusty rim was left of the
5-inch Rola speaker.
This shows the chassis after it has been painted, labels added and the
loudspeaker replaced. Note the gaffer tape around the base of the first IF
transformer, hiding a large hole. It’s still a pile of garbage.
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Celebrating 30 Years
But apart from that, most of the
components were still there, even
though none of them would ever operate again!
As a start, the bare case was thoroughly washed and the lettering “HIS
MASTER’S VOICE” emerged from
underneath the encrustation of mud.
The case had some serious fracturing
but fortunately, a large fragment of the
missing top section was present as a
separate piece. So it was not beyond
redemption.
The next step was to cut an aluminium sheet to span the gap, large enough
to overlap so that it could be clamped
and glued in place with Araldite from
below.
This corrected the distortion of the
case and provided a base for gluing the
large fragment. Then 2-part car body
filler was applied to achieve a good
surface for sanding back.
This was followed with an undercoat, then a spray with Motortechbrand “Indian Red” paint. The result
was similar to the appearance of the
original Bakelite case and certainly a
miraculous improvement over the initial condition. Some yellow speaker
grille cloth and knobs completed the
external restoration.
Painting over the defects
The rust-encrusted steel chassis was
cleaned up as well as possible but not
too vigorously because it was tissue
thin in many places. A coating of silver paint (water-based acrylic enamel)
restored the appearance of the chassis.
A little paint hides a lot defects; well,
more or less.
And even though the radio would
never be operational, a replacement
5-inch Rola speaker was essential to
keeping up appearances.
The original phenolic panel for the
aerial and earth connections simply
crumbled away due to the adverse effects of water immersion, so I fitted a
new set of terminals.
A glance at the photos of the chassis
before and after this will reveal the full
extent of this superficial restoration to
a “static model”. Notice the exposed
windings of the primary transformer
after it had been hosed off.
Perhaps the most remarkable aspect
of this story was the screen-printed
dial. Once the caked-on mud had been
carefully cleaned off, all the station
markings were there in their original
condition.
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Apart from rising to the challenge
of restoring this model as a roughand-ready static model and thereby
attempting to maintain my reputation
as the patron saint of restorers, er, lost
causes, what is the particular interest
of this HMV 5-valve superhet radio?
The model C13C is quite similar to
mantel radios offered by other manufacturers at the time.
Released in 1951, it has its legacy
in the 1940s, both electronically and
by way of styling. As an end-of-era example, it merits a place in the history
of Australian radio.
Only one IF stage
There are two noteworthy aspects of
the circuit shown in Fig.1. First, there
is only one IF amplification stage and
the tone control is not the usual continuously-variable top-cut type but is
a 3-position switch, with “Bass” and
“Speech” settings.
On the left-hand side, we see a conventional aerial coil in two sections
and a 3-pole switch (S1) provides the
This shows the tarted-up chassis back in the newly painted cabinet. The tuning
capacitor for this radio was not replaced as the area where it would sit is still
heavily corroded. The 6V6 tetrode output valve (far right) was coloured black
using a marker pen to hide the fact that it was gassy.
Fig.1: the circuit is quite conventional except that it does not have a variable tone control but a 3-position switch giving
“Bass” and “Speech” settings.
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January 2018 91
Shown above is the freshly painted cabinet and finished restoration for
the HMV C13C mantel radio. From left-to-right, the first knob is the band
switch, next is tone control and the last is for volume/power.
While not the star of this article, this HMV E43E radiogram used a nearly
identical circuit as the HMV C13C mantel radio but was still regarded as
hifi. The only difference was that it had a 12-inch Rola loudspeaker.
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Celebrating 30 Years
band switching: medium wave, MW
ranging from 540kHz to 1600kHz or
shortwave, 16.5 to 51 metres (5.9 to
18.2MHz).
Band switch S1 selects the appropriate secondary winding of the aerial
coil to be tuned by the first gang of the
tuning capacitor and also selects the
coils for the local oscillator.
The only miniature valve in the
chassis is the 9-pin 6AN7 as the frequency converter (mixer-oscillator).
The other valves are classic octal types
(ie, 8-pin with a Bakelite base) with a
heritage dating back to the 1930s.
The 6AN7 was released by Philips,
Eindhoven as the ECH80 for Europe
in March 1949. This 9-pin miniature
valve then became a common inclusion for Australian radios of the 1950s.
It required 6.3V for the filament at
230mA.
The intermediate frequency (IF) of
this set is 457.5kHz. This was fairly
common for HMV sets around this
time, but most sets of this era would
have had a 455kHz IF.
The dial calibration is almost entirely devoted to the MW band which
suggests that casual domestic listening
was the primary market.
The band change switch also
selects the gramophone pickup. When
the pickup input is selected (switch 1
position 1), the local oscillator coils
are disconnected.
This disables the tuner section to
avoid the potential for annoying breakthrough of radio while playing records. The gramophone pickup feeds
in through the two central sockets at
the rear of the chassis.
It seems the HMV C13C was rarely used with a pickup. In reality, the
gramophone pickup connection was a
standard feature of the chassis which
HMV did use in a wooden cabinet radiogram, model E43E of 1951.
After the 6AN7 frequency changer,
the secondary of the first IF transformer drives the grid of the 6AR7 amplifier-demodulator valve. This valve was
designed and manufactured by the
Amalgamated Wireless Valve Company (AWV). Rather than a typical twindiode tetrode IF amplifier such as the
6N8, the 6AR7 is a pentode partnered
with twin diodes.
The pentode’s higher gain compensates to some extent for the lack of a
second IF valve. The pentode’s plate
drives the second IF transformer and
there is adequate signal to pass to a
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Table 1: voltage table for the valves used in the HMV C13C and various other
HMV radio sets.
diode in the 6AR7 for demodulation.
The recovered audio then passes
to the volume control potentiometer
which is ganged with the mains on/
off switch, a common feature of sets
of this era. The second 6AR7 diode
generates the AGC voltage which is
fed to the grids of both the 6AN7 and
6AR7 to reduce gain for high strength
signals.
The following 6J7 audio preamplifier pentode and 6V6 beam-tetrode
output valve provide an audio section
that is capable of producing around
3W from either a crystal gramophone
pickup or local radio signals.
Tone control is by the aforementioned 3-position switch (S2). Its
“Bass” setting is simply a top-cut
provided by a 1nF capacitor and the
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“Speech” position is bass-cut by adding a series 0.5nF capacitor to the signal path.
The circuit was basic to several
HMV models as can be seen from the
voltage table (Table 1) reproduced here
that lists multiple models. The table
clarifies the function of each valve pin,
as well as giving operating voltages
and current.
Using the same circuit and chassis
as in the C13C, the HMV E43E radiogram was “hifi” in 1951. The only difference was the provision of a well-baffled 12-inch Rola speaker to provide
good volume and frequency response.
The HMV radiogram pictured here
from the author’s collection shows
that it was also an elegant item of
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